a Directory of Minimally Invasive Spine Surgery, Department of Neurosurgery, Chicago Institute of Neurosurgery and Neuroresearch, Rush-Presbyterian St. Luke's Medical Center and 0 Division of Neurosurgery, University of Chicago, Chicago, Ill., USA

Clinically significant thoracic disc herniation is a relatively rare condition with an estimated annual incidence of approximately one per million. Surgical management of thoracic disc herniation comprises between 0.15 and 4% of all operations for herniated discs including those of the cervical and lumbar spine [3, 40, 41].

Several surgical techniques have been developed to approach the thoracic spine including costotransversectomy, lateral extracavitary, lateral parascapular extrapleural, transfacet, transpedicular, and transthoracic approaches [1, 17, 41]. Although all of these approaches have been successful in reaching thoracic pathology, each procedure requires a relatively large skin incision and extensive bony work that subsequently may lead to considerable postoperative morbidity [4, 12, 37, 41].

Outcomes from the traditional posterior approach, thoracic laminectomy, to treat thoracic disc herniation are especially poor with large percentages of patients failing to improve or, more significantly, deteriorating further. Although anterior approaches facilitate exposure to the ventral aspect of the thecal sac, they require entry into the pleural cavity and place the intrathoracic contents at risk. Posterolateral approaches, especially the lateral extracavitary one, provide many of the benefits of an anterior approach without the additional risks of entering the thoracic cavity. Unfortunately, the exposure for this approach requires extensive muscle dissection that adds to the postoperative morbidity.

Perot and Munro [33] and Ransohoff et al. [34] performed the initial transthoracic approach to thoracic disc herniation after which the transthoracic approach became a standard procedure to manage thoracic disc herniations. This approach provides excellent visualization of the ventral aspect of the thoracic spine without the risk of spinal cord manipulation. However, the thoracotomy used in this approach requires a large skin incision, extensive lung and rib retraction, and muscle dissection, all of which can contribute to postoperative pulmonary dysfunction, pain and increased morbidity.

To reduce the morbidity associated with the thoracotomy technique and other approaches previously mentioned, less invasive thoracoscopic techniques have been developed and more recently refined for the treatment of herniated thoracic disc [1, 8, 9, 35, 36]. Thoracoscopy is capable of producing the same exposure as the transthoracic route without the need for a large thoracotomy incision. Thoracoscopy was introduced by Jacobeus in 1910 [19]. However, this technique was not widely adopted at that time due to poor illumination of intrathoracic structures and other technical limitations. It was not until the 1970s that the modern era of surgical endoscopy began following a number of technical advancements such as fiberoptics, and the operating endoscopes. Introduction of the video camera into thoracoscopic surgery was the next major advance leading to the introduction of the video-assisted thoracoscopic surgery technique [14, 18, 19, 35]. Thoracoscopic discectomy is an effective alternative to traditional open thoracotomy techniques that has been found to reduce the incidence of pulmonary morbidity [11, 13], intercostal neuralgia, and shoulder girdle dysfunction [13, 20]. Although the incidence thoracoscopic complications may vary they are similar to those encountered with open thoracotomy [16, 30].

Although the procedure-associated morbidity is much less than with thoracotomy, the prevalence of pulmonary complications such as postoperative atelectasis, pneumothorax, pleural effusion, and hemothorax is considerable [7, 10, 22, 37, 39]. In addition, thoracoscopic surgery is technically demanding and requires attainment of new surgical skills. Therefore, it is advised that surgeons perform the thoracoscopic procedures only after pursuing appropriate training that includes extensive practice of this skill in a surgical laboratory [4]. Even today the use of video-assisted thoracoscopic surgery techniques to treat thoracic disc herniation have not been widely adopted due to the steep learning curve which requires specialized training to master.

We have developed a novel posterolateral, minimally invasive thoracic microendoscopic discectomy (TMED) technique that potentially provides a less morbid approach to the treatment of thoracic disc herniation. The procedure is performed via a posterolateral approach with the patient in the prone position (fig. 1a) and therefore avoids entry into the thoracic cavity. A number of studies have shown that minimally invasive spine surgery is associated with less disruption of normal tissue, less blood loss, and less procedure time than traditional open procedures, which translates to patients experiencing reduced postoperative pain, reduced hospitalization, and reduced recovery time [1, 4, 15, 32]. Based on our validation data from cadaveric studies we have demonstrated that this technique could be safely implemented into clinical use. The technique is a modification of the lateral extracavitary approach, which was a further modification of the lateral rachiotomy initially performed in 1961 for the treatment of Pott's disease by Larson et al. [21]. Unlike the traditional lateral extracavitary approach which requires significant muscle, rib head and transverse process excision to reach the thoracic disc, the TMED approach avoids much of this dissection. It uses a series of muscle dilators, a tubular retractor and endoscope that was initially developed to approach lumbar herniated disc pathology [31]. Herein we report this novel minimally invasive technique to treat herniated thoracic disc pathology.

Indication

The clear indication to perform the TMED procedure is in patients with thoracic disc herniation causing myelopathy in order to improve neurological function and prevent further injury to the spinal cord. However, the longer the myelopathy has been present the less likely is restoration of normal neurological function. Severe and refractory radiculopathy is another indication for surgery. Thoracic radicular pain from disc herniation is typically sharp, lancinating pain that radiates unilaterally from posterior to anterior around the chest wall in its dermatomal distribution correlating to the herniated thoracic disc. Radiculopathy can develop without loss of neurological function because of the overlap of innervation of adjacent intercostal nerves. Radiculopathy may be present without myelopathic manifestations. Isolated thoracic radiculopathy often responds to conservative management such as nonsteroidal anti-inflammatory medications, oral steroids, epidural injection, intercostal nerve injection, activity modification, and hyperextension brace [41]. Pain relief after thoracic discec-tomy is usually favorable in patients with radiculopathy, even for those with long-standing radiculopathy. Although it is controversial whether patients with localized back pain and axial pain without myelopathy require surgery or not, most surgeons do not recommend thoracic discectomy for isolated back pain unless it is associated with a neurological deficit [28, 29, 37, 38, 40]. This technique is not recommended for a large calcified midline thoracic disc that may require an anterior approach.

Technique

Posterolateral Microendoscopic Discectomy

We have developed a novel posterolateral, minimally invasive TMED technique that potentially provides a less morbid treatment for thoracic disc herniation compared to traditional techniques. This method has been used successfully to treat lateralized and central thoracic disc herniations causing radicular and myelopathic symptoms. This technique was developed with similar instruments used to perform a microendoscopic lumbar discectomy using the METRx system of tubular dilators and retractor initially developed by Smith and Foley, see Perez-Cruet et al. [31]. The procedure is performed as follows.

Patient Positioning and Operative Setup

After general endotracheal induction and placement of a Foley catheter, the patient is positioned prone on a radiolucent Wilson frame. All pressure points are padded adequately and the arms are placed above the patient making sure not to overextend at the shoulder beyond 90° (fig. 1a). Somatosensory evoked potentials are measured throughout the procedure. Any change in somatosensory evoked potentials during the procedure warrants investigation as to the probable cause. The fluoroscopic C-arm is positioned to allow for a lateral x-ray image of the operative area. A video monitor is placed opposite the surgeon to allow for a clear field of view of the monitor. The back is shaved and prepped in a routine surgical fashion.

Under fluoroscopic guidance a spinal needle was used to locate the level of the herniated thoracic disc. The exact location of the thoracic disc herniation was confirmed by counting from the sacrum below and/or from the occiput above. Once the correct location was identified, a Steinmann pin was placed at the medial aspect of the transverse process at the level of the thoracic disc herniation. An incision of approximately 2 cm was made 3-4 cm lateral to the midline, depending on the size of the patient, through which a series of tubular muscle dilators was placed (fig. 1b, c). A tubular retractor was placed over the final muscle dilator and then affixed to a flexible arm secured firmly to the operative table (fig. 1d). The endoscopic assembly was then focused, white balanced, and passed down the center of the tubular retractor (fig. 1e). Once the endoscope was in place the orientation of the endoscopic image seen on the monitor was adjusted so that medial anatomy was at the top of the monitor screen, lateral anatomy at the bottom, and rostral and caudal anatomy was in the same orientation as the patient on the operative table. This greatly facilitated the procedure and aided in orienting the surgeon throughout the procedure. Surgical orientation throughout the case is of upmost importance to prevent inadvertent entrance into the spinal canal and this is facilitated by lateral and AP fluoroscopic images to help confirm the location of dissection. The proper level was then reconfirmed using lateral fluoroscopy and the TMED procedure was performed under endoscopic visualization. The muscle and soft tissue overlying the proximal portion of the transverse process and lateral facet complex were removed using an insulated Bovey cautery. Careful probing with a ball-ended probe helps in defining the bone margins and liberal use of fluoroscope imaging also helps to orient the surgeon thoroughout the procedure. The approach is similar to that of a transforaminal approach and adequate laterality of the approach reduces any manipulation of the spinal cord during disc removal. Access to the disc space required removal of a small amount of bone. This is facilitated by the use of a long tapered high-speed drill. Once the bone architecture is defined a small portion of the proximal rib head and transverse process and a portion of the lateral aspect of the facet complex are removed. This procedure facilitated widening of the neural foramen. The thoracic pedicle of the more caudal vertebral body was identified and followed to the appropriate disc level. The disc annulus was identified and the epidural veins coagulated with a bipolar cautery. The annulus was cut with a thin anulotomy knife and the discectomy was performed. The lateral projectory of the tubular retractor through a transforaminal type approach allowed for extensive disc removal under the thoracic spinal cord and dura enabling even midline placed thoracic disc herniations to be removed (fig. 2a, b). Following complete discectomy further inspection underneath the dura and spinal cord using a Woodson elevator or similar instrument assured complete decompression (fig. 1f). The tubular retractor was removed and a suture was placed in the thoracodorsal fascia. Interrupted subcuticular sutures were placed and the skin closed with Dermabond.

This technique has a number of advantages over other traditional techniques used to perform thoracic discectomy and includes the following: avoidance of traversing through the thoracic cavity, maintaining the integrity of the disc, avoidance of the need for thoracic fusion, and avoidance of extensive

Fig. 1. a Positioning of the patient in the prone position on a radiolucent frame with lateral fluoroscopy for performing TMED procedure. After placing a K wire on the transverse process (b, c) a series of muscle dilators are used over which a tubular retractor is placed (d). e The operation is then performed under endoscopic visualization. f A Woodson elevator instrument is useful in removing a central disc herniation.

Fig. 1. a Positioning of the patient in the prone position on a radiolucent frame with lateral fluoroscopy for performing TMED procedure. After placing a K wire on the transverse process (b, c) a series of muscle dilators are used over which a tubular retractor is placed (d). e The operation is then performed under endoscopic visualization. f A Woodson elevator instrument is useful in removing a central disc herniation.

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